Electrophotographic developing process

ABSTRACT

An electrophotographic cascade developing process and apparatus for rendering an electrostatic latent image visible which comprises (1) providing a developing electrode in the proximity of a surface holding the electrostatic latent image to be developed thereon and in facing relationship to the surface; (2) moving the surface with respect to the electrode; (3) developing the latent image by flowing cascade developer including charged toner particles and carrier particles between the surface and the electrode while applying to the electrode a first electric potential of a polarity capable of attracting the toner particles; and (4) after the portion of the surface containing the developed image is moved past the space under the influence of the electrode, changing the first potential on the electrode to a second potential which attracts said charged toner particles less than the first potential while maintaining the flow of developer to thereby remove toner particles sticking on the electrode.

Jan. 8, 1974 8/1971 Satomi................,,..............118/637. 10/1971 ELECTROPHOTOGRAPHIC DEVELOPING 3,611,982 Coriale 118/4 PROCESS [75] Inventors: Katsuo Makino, Odawara; Seiji Joh; Primary Examiner Mervin Stein Kwzo Takeda both of Tokyo an Assistant Examiner-Leo Millstein of Japan Att0rney-Geralcl J Ferguson, Jr. et a].

Ltd., Tokyo, Japan [73] Assignee: Fuji Xerox C0.,

[22] Filed: Dec. 23, 1971 21 Appl( No.: 211,577

and apparatus for rendering an electrostatic latent image visible which comprises (1) providing a devel- [301 Foreign Application Priority Data oping electrode in the proximity of a surface holding Dec 26 1970 Japan the electrostatic latent image to be developed thereon and in facing relationship to the surface; (2) moving the surface with respect to the electrode; (3) developmg the latent image by flowing cascade developer including charged toner particles and carrier particles between the surface and the electrode while applying to the electrode a first electric potential of a polarity 48 5 6 7 r 3 m37 6 36 G1 a 81 7 180 .h 7 7 7 7 8D 16 11/ 1M 12 1 m l m "6 m mm4 "m2 m mmm rD u na/ 8 L L 1 C 10 s .m to. 0 I. U MP 2 oo 5755 l. [l

capable of attracting the toner particles; and (4) after the portion of the surface containing the developed [56] References Cited UNITED STATES PATENTS the electrode, changing the first potential on the elecr mm e a r. 0 m

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2. Description of the Prior Art In general, electrophotographic processes employ a photosensitive element consisting of a photoconductive layer disposed on an electroconductive support material. In the most common process, the reproduced image is obtained by uniformly charging the surface of the photosensitive element electrostatically and successively subjecting the surface to imagewise exposure thereby, dissipating the electrostatic charge in the exposed area in accordance with the intensity of light to obtain a distribution pattern of electrostatic charge on the surface.

The distribution pattern of electrostatic charge, or the electrostatic latent image, can be rendered visible,

or be developed by bringing the latent image intocontact with electrostatically charged colored powdered material. The powdered material deposits on the (hereinafter referred to as toner) and coarse particles (hereinafter referred toas carrier). The carrier is generally' composed of particles of glass, sand, steel etc. covered with other suitable material so as to be distant from the toner in the series of frictional electricity. Consequently toner and carrier mixed together generate electrostatic charges of opposed polarities and attract each other electrostatically. When the mixture of ton'er'and carrier is brought into contact with the surface of the photosensitive element holding the electrostatic latent image thereon,the toner leavesthe carrier and deposits on the image area by means of the electro static attractive force of the electrostatic charge on the surface of the photosensitive element thereby render ing the latent image visible.

A wide variety of methods has been proposed and utilized to develop the electrostatic latent image by contacting the colored powdered material with the surface of the photosensitive element. Among these methods is the widely accepted, so-called cascade developing method" in which development is performed by flowing developer consisting of toner and carrier along the surface of the photosensitive element by means of gravity, whereby the toner is attracted by the electrostatic charge present on the surface of photosensitive element an attractive force stronger than that of the carrier. The cascade method has already proved to be cess to a certain extent. For example the use of an elecan extremely economical advantageous process capable of reproducing line images with a very high reproduction quality.

On the other hand the conventional cascade process is incapable of faithfully reproducing original image containing blackout areas or continuous tones. Thus, it is known that the cascade process is incapable of uniformly developing a large unexposed area. Instead, a higher density at the peripheral portion of the area is obtained than at the central portion thereof. The inability of the cascade process to develop a blackout area is attributed to equilibrium between essentially two competing factors, one being the action of the toner particles to adhere to the image area and the other being the action of the toner particles to cleave from said area. The deposition of toner particles is accelerated by intensifying the strength of electrostatic field extending from the electrostatic latent image away from the photosensitive element. The strength of the electrostatic field in the peripheral portion of the image area is generally stronger than the attractive force of the carrier for the toner.

The removal of toner once deposited is caused by the attraction between toner and carrier. Such removal of once deposited toner by means of the carrier will hereinafter be referred to as the cleaning effect. The cleaning effect, though it removes a part of the deposited toner from the central portion of image area, is extremely useful for removing unnecessary toner sticking on the non-image area, since such toner is bound quite loosely with the surface.

In the conventional process, because of these two mutually competingfactors, it has been necessary to carefully control the mixing ratio of toner and carrier.

The use of developer with excessively high toner content will lead to undesirably high fog density and decrease of the cleaning effect, whereas theme of devel oper with excessively low toner content will inevitably result in low reproduction density and undesirably strong cleaning effect. Although it has been practically possible to find a suitable balance between these two factors by means of control on toner content in case of line reproduction, such control is not effective when reproducing an original image containing blackout areas or continuous tones.

Various attempts have been made to remove this drawback of the cascade developing processwithsuctrode, which is known as developing electrode, in the proximity of the surface holding the electrostatic latent image is extremely effective for development of a blackout area. The developing electrode generates a strong electric field between the electrode and the surface holding the latent image, which field promotes the attraction of toner particles by the electrostatic latent image, resulting in larger deposition of toner as said the electrode and the surface holding the latent image of the developer in the space between the developing electrode and the latent image holding surface. Such clogging, if once started, will not only contaminate the toner already deposited on the latent image holding surface but eventually will also lead to mechanical destruction of the surface. In the prior developing electrode process, this fact restricts the amount and speed of developer blowing down on the surface of the latent image holding surface and accordingly poses a limit on the developing speed. Stated differently the processing speed of the reproducing device has had to be lowered in order to obtain satisfactory image quality because of the reduced flow rate of the developer to prevent the aforementioned clogging thereof. Various attempts made to prevent developer clogging, but it cannot be said that they have been successful enough. For example, the developing electrode composed of metal wires or provided with openings inevitably results in reduced developing efficiency. Also an attempt to improve developing efficiency with prolonged development with a larger developing electrode is considered inadequate whenever spatial restrictions exist within the reproducing device.

Another drawback of prior art cascade developing processing using a developing electrode is that a deterioration of image quality gradually occurs due to stain on the developing electrode. That is, the developing electrode is applied with suitable electric potential of appropriate polarity to remove undesirable toner (fogging) deposited on the non-image area, and it therefore attracts toner. This results in reduced effect of the applied potential and gradual increased fogging. Thus, the amount of toner sticking on the developing electrode as well as fogging increases with time.

SUMMARY OF THE INVENTION In order to remove the drawbacks of the abovementioned conventional processes, there is provided a novel developing process of; 1. flowing cascade developer by means of gravity along a slanted surface provided with an upwardly directed latent image; 2. providing a pool for developer thereby realizing direct contact of said developer with a part of the surface and; 3. further providing a developing electrode below said pool and in facing relationship with the surface. In this process the developer is supplied from above the pool, and the developer in the pool drops through the space between the developing electrode below the pool and the surface holding said latent image thereon. The latent image holding surface is displaced with respect to the pool and developing electrode to realize development in consecutively displaced portions of the surface. The developer in the pool is maintained at a constant performance level by replenishment from above. Thus the development is carried out as described above at the pool and the developing electrode.

Thus, the primary object of this invention is to provide a novel developing process and apparatus free from the drawbacks mentioned above associated with the prior art developing processes and devices.

Another object of this invention is to provide an im proved cascade developing process and apparatus using a developing electrode and developer consisting of two components, namely toner and carrier, and being capable of obtaining high quality reproduction from images containing blackout areas or continuous tones.

Still another object of this invention is to provide an improved cascade developing process and apparatus using a developing electrode and capable of preventing the undesirable deposition of toner in the non-image area on the surface holding the electrostatic latent image thereon.

Further another object of this invention is to provide an improved developing process and apparatus capable of providing a high density image under high speed developing conditions.

Further another object of this invention is to provide an improved cascade developing process and apparatus capable of inhibiting the clogging of developer.

Further another object of this invention is to provide an improved cascade developing process and apparatus capable of providing high developing efficiency in a limited developing space.

Further another object of this invention is to provide a process and apparatus for effectively removing stain by a developer from the developing electrode automatically and without requiring a complicated mechanism.

Further another object of this invention is to provide a developing process and apparatus capable of inhibiting fogging to a tolerable level in continuous multiple copying.

Other objects and advantages of this invention will become apparent upon reading the appended claims in conjunction with the following detailed description and the attached drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a schematic cross section of an illustrated device for use with the developing process according to this invention.

FIG. 2 is an illustrative circuit for controlling the voltage applied to the developing electrode in the device of FIG. 1.

FIG. 3 is a graph showing the time-dependent change of development fog density in a continuous development process.

GENERAL DISCUSSION OF INVENTION In the process of this invention, the development is carried out in the presence of aforementioned developer pool and developing electrode. The developing electrode is disconnected from the potential applied thereto shortly after the completion of development and grounded or connected to a potential of inverted polarity. The developer then flows for a short period to thereby clean the developing electrode and prevent the excessive deposition of toner thereon.

The process of this invention can be applied, for example, to an electrophotographic photosensitive element composed of an insulative photoconductive layer provided on the external periphery of a cylindrical electroconductive support having a horizontal rotating axis. The electrostatic latent image to be developed is formed on the external peripheral surface of the electrophotographic photosensitive element. In the following explanation it is assumed that the photosensitive element is rotated anticlockwise during the development. In this case the developer pool is provided in the upper part of the second quadrant, while the developing electrode is provided in the lower part of second quadrant and below the developer pool, as is brought out in more detail hereinafter. The fresh developer supplied above the pool is brought into maximum contact with the latent image holding surface in the pool with relative movement therebetween. The carrier contained in the developer also performs an approximate rotating movement. These three factors cooperate to obtain developing efficiency at a miximum level.

The efficiency of the cascade development is governed by the frequency of contact or collision of the carrier holding toner with the latent image holding surface. Stated more specifically the higher frequency provides higher efficiency to thereby provide high image quality within a shorter developing time or within a shorter or narrower developing space. Further, if the nuclei of the carrier particles are composed of electroconductive materials, an effective developing electrode is provided at an extremely short distance from the latent image holding surface to be developed, since the carrier is forced to flow in a highly packed state in the developer pool. Consequently the image is developed with an extremely high density while moved with respect to the developer pool and the blackout areas are faithfully reproduced. In addition, it is possible to obtain a developed, continuous tone image, the density of which corresponds to the charge density of the latent image on the surface to be developed. Such a continuous tone image of high quality is obtained when the latent image is passed through a limited developing space within a short developing time.

As explained hereinbefore a development process of high efficiency which is capable of reproducing blackout areas inevitably causes certain undesirable deposition of the toner in the nonimage areas. Thus, when the surface to be developed passes under the developing electrode, certain undesirable deposition of toner occurs in non-image areas'fln cascade development, toner particles are generally separated from the carrier or the surface to be developed because of mutual collision of the carrier particles or the collision of carrier particles with the surface to form a powder cloud which sticks on the non-image areas to cause undesirable fogging. The generation of such powder cloud in the developer pool according to this invention is limited very much but cannot be avoided completely. According to this invention, the undesirable powder cloud of toner is removed by the developing electrode provided below the developer pool. The developing electrode has applied thereto a high electric potential to intensify the cleaning effect and to effectively remove the toner undesirably deposited in the non-image areas. Though the developing electrode can be effective even when provided in the third quadrant of the cylindrical support where the flow of cascade developer is separated by gravity from the surface to be developed, much better results are obtained in the second quadrant where the cascade developer flows in contact with the surface to be developed by means of gravity. Thus, in summary, in the process according to this invention, the development of high efficiency is realized while the surface to be developed is moved while in contact with the developer pool while the undesirably deposited toner is removed during the passage under the developing electrode to thereby obtain a developed image of high quality. v

. in the above case the developing electrode normally has applied thereto an electric potential inverted with respect to the polarity of electrostatic charge on toner particles. The toner particles inevitably stick to the developing electrode because of the potential of inverted polarity applied to the electrode. Thus, positive potential may be applied to the electrode when negatively charged toner is used. The toner thus deposited on the electrode reduces the strength of the electric field produced by the electrode between the surface of the photosensitive element and the developing electrode and decreases the practical effect of the developing electrode. Thus it becomes necessary to remove the deposited toner automatically. The toner can be removed by flowing the developer as in the case of development but with the developing electrode grounded or connected to an electric potential having the same polarity as the toner. Thus, after completion of development, the toner is removed by grounding the developing electrode or inverting the polarity of the potential applied thereto. Such removal can be completed within an extremely short time.

In the process of this invention it is possible to employ any already known two-component developer consisting at least of toner and carrier, provided generally with particle sizes of l 30 um and 50 800 pm, respectively.

The process and apparatus of this invention obtains results comparable to those obtainable with conventional developing processes utilizing a developing electrode, without causing the phenomenon of developer clogging frequently experienced in the conventional processes. No developer clogging occurs in the developer pool where the development is carried out at a high efficiency because open space is provided at least above the developer. Further, it is not necessary to minimize the distance between the surface to be developed and the developing electrode since theelectric field occuring in the space between the surface and the developing electrode need not be faithfully correspondent to the electrostatic latent image as the electrode may not be provided primarily for improving the developing effect but rather for eliminating undesirably deposited toner. Consequently the expected cleaning effect can be obtained by maintaining the distance at a value so as not to disturb the free flow of developer.

High efficient development is made possible by the direct contact between the developer pool and the surface to be developed and is realized by the exposure for a short time of the surface to be developed to the zone (developer pool) where the development is carried out (hereinafter referred to as developing zone). The developing zone should have a minimum size. When the speed of the surface to be developed, or the photosensitive element, is increased, the developing zone should be made larger since the period during which the surface is in said developing zone or is in contact with said developer pool is decreased. The minimum size of the developing zone is that dimension of the developer pool which permits relatively free flow of developer resulting from mutual friction with the surface to be developed and from the weight of developerbut other than the flow toward the developing electrode provided thereunder without the clogging of developer. The minimum size is generally equal to the depth of said developer pool or larger though the minimum size will also vary depending on the relative speed between the developer and the surface to 'be developed, as discussed above. I

Whenever it is desirable to obtain a positive reproduction from a positive original image on a positively charged 'photosenstive surface, a negatively charged toner is used. In this case the developing electrode is supplied with a positive potential in order to prevent undesirable deposition of toner in the non-image area, thereby forming an electric field in the space between the electrode and the surface of photosensitive element which attracts toner to the electrode. The electric field is gradually weakened with the increase of toner deposited on the electrode thereby decreasing the effect of the electrode to suppress the fogging. Consequently it becomes necessary to remove the deposited toner periodically or at suitable times. This can be realized, for example, when the development is completed and the surface to be developed is no longer present in the aforementioned developing zone, by bringing the developing electrode to the identical potential as that of the photosensitive element or by inverting the electric field between the photosensitive element and the developing electrode by means of applying negative potential thereto in the foregoing example. The deposited toner can be removed to a certain extent by this operation, but further effective removal can be realized by flowing the developer in the space between the electrode and the photosensitive element. It is therefore desirable to continue the operation of developing device to maintain the flow of developer even after the potential applied to the developing electrode is inverted. In the process according to this invention, a certain amount of developer stored in the aforementioned developer pool keeps running down in the space between the electrode and the photosensitive element after the operation of developing device is terminated and thus the above-mentioned result is obtained even if the aforementioned inversion of the electrode potential is effected simultaneously with the termination of the operation of the developing device.

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 is an illustrative schematic cross section of a device capable of effecting the improved developing process of this invention. An electrophotographic photosensitive drum 1 is rotated anticlockwise and is composed of a metallic drum 12 and a layer 11 of insulative photoconductive material such as amorphous selenium provided thereon. The surface of drum l is uniformly charged by a conventional corona discharging device 2, and then subjected to image exposure in an exposure station 3, in a manner well known to those of ordinary skill in this art. An electrostatic latent image is thus obtained on layer 11 and developed by a developing device generally indicated at 4.

The drum 1 is kept in contact with developer 15 at a developer pool 13 to form a developing zone which is a part of the second quadrant of the periphery of the drum represented by 6 in the drawing. The developer is transferred upwardly by a bucket elevator 16 and continuously supplied to the developer pool 13, and then flowed downwardly therefrom by gravity through the space between a developing electrode 14 and the drum 1.

The developer undergoes rather complicated movement in pool 13. When the drum 1 is still, the developer in pool 13 simply flows downwards with a speed much slower than the flow speed in the space between the developing electrode 14 and the surface of drum. When the drum is rotated, the speed of the developer in the pool is slower than the peripheral speed of said drum. Consequently the developer in the pool 13 moves as indicated by the arrow A due to friction with the surface of the drum. This complicated movement is apparently caused by a combination of the two above-mentioned movements. A side wall 19 for the pool 13 is composed of an electroconductive material, which can be supplied with direct current electric potential. The surface of said side wall 19 or a part thereof coming into contact with the developer is preferably covered with thin layer of insulative synthetic resin.

The developer flowing out from the developer pool 13 is forced to pass through the space between the developing electrode 14 and the surface of the drum. The developing electrode 14 has applied thereto a direct current electric potential by means of a potential source (not shown) to collect toner deposited on the non-image areas of the drum surface. The collected toner is stored, together with falling developer, in the bottom 17 of the developing device 4, from which the developer is cycled to the developer pool 13 by means of bucket elevator 16. Consumption of toner is replenished automatically by the toner supplier 18.

The developed image coming out from said developing device 4 is transferred, by means of a transferring corotron 6, onto a transfer sheet 7 which is moved synchronously with the periphery of the surface of drum 11. The powder image thus transferred is fixed in a fixing device 20 to provide permanent reproduction. The drum 1 is cleaned by removing the toner particles remaining thereon at cleaning station 9. Further drum 1 is subjected to flush exposure in a flush exposure station 10 to dissipate remaining surfacial charge thereby completing one cycle of the process.

The developing electrode 14 can be made of any electroconductive material so long as the electrode can be positioned at a predetermined distance from the surface of drum 10. Further, the surface of the electrode can be covered with thin layer of insulative synthetic resin. The developing electrode 14 and the side wall 19 of the developer pool are electrically insulated from one another so that different electric potentials can be applied thereto. The potential applied to the developing electrode is preferably higher by 50 to 1,000 volts than the surface potential in the non-image areas on drum 1, this depending on the distances between the developing electrode 14 and the surface of drum 1. The potential should be of same polarity as that of the electrostatic latent image, though ground potential or potential of inverted polarity can also be utilized according to the kind and condition of development. In general, however, the potential applied on the developing electrode should be of inverted polarity with respect to the charge on the toner (namely negative or positive potential for positively or negatively charged toner, respectively).

When the development is continued without cleaning the developing electrode the fogging level gradually increases as shown in FIG. 3, which indicatesthe number of copies (proportional to total developing time) along the abscissa and the amount of toner deposited on the electrode and fogging on the copy in the ordinate. The amount of deposited toner gradually increases as shown by the solid line and reaches saturation of about 0.35 at about 500 copies. On the other hand, the fogging level (represented by the fogging after transfer to the transfer sheet) shows an increase from 0.01 to 0.02 in reflective optical density and reaches saturation at around 500 copies. This indicates that deposition of The application of electric potential is preferably realized as follows. During the copying operation in known electrophotographic copying devices, the exposure system and developing system are operated simultaneously. The device still is in operation for a short time for transferring the image to the transfer paper and so forth even after the copying is completed and the exposure and developing systems are switched off. In such known copying devices the switchover of potentials applied to the developing electrode can be effected by the signal terminating the operation of exposure system while the developing system can be maintained in operation for several seconds until the entire copying device is stopped. Also it is possible to terminate the operation of the developing system simultaneously with the switchover of potential thereby effecting the removal of deposited toner by spontaneous flow of developer stored in the hopper or in the developer pool. The potential applied to the developing electrode can be inverted again by the signal starting the exposure or developing system. Such inversion of potential is not limited to the above methods but can be realized by various signals or timers according to the mechanism of the particular copying device.

The process of this invention has thus far been explained in an embodiment thereof comprising a developer pool at the upper part of developing station, it has experimentally been confirmed that the process can be employed in the developing device excluding the developer pool so long as the developing electrode is pro-' vided at the last part of developing section.

This invention will be further clarified by the following example which is intended to clarify the effect of a mixed developing process according to this invention consisting of; 1. a developing zone of high efficiency and; 2. cleaningzone lt is to be noted that the following example is merely illustrative of various preferable means for effecting this invention and is not intended to limit .theinvention in any way.

EXAMPLE An electrophotographic photosensitive drum of diameter of about 200 mm (composed of amorphous, 50 um selenium layer formed by vacuum evaporation on the periphery of an aluminum drum) is uniformly charged by corona discharge to a positive potential of about 700 volts, and then subjected to imagewise exposure to form an electrostatic latent image thereon. The electrophotographic photosensitive drum holding said electrostatic latent image thereon is moved through a cascadedeveloping device as shown in FIG. 1 consisting of a developing zone in which the drum is brought into direct contact with a developer pool and a cleaning zone containing an electrode disposed in facing rela tionship with the 'drum'. The cascade developer supplied to said developer pool consists of a mixture of 1 part by weight of negatively chargeable toner particles of a black-colored polystyrene copolymer about um in diameter and 300 parts by weight of steel ball carrier about 400 pm in diameter covered with a thin resin layer which produces a negative charge on the toner by friction therewith. The developer is supplied by a bucket elevator to the developer pool so that the pool is constantly filled with the developer to at least percent of the maximum capacity thereof. The longitudinal length of the direct contact zone between the developer pool and the photosensitive drum is about 50 mm, while the distance between the side wall of the pool and the drum surface is about 30 mm. The side wall as well as the drum are electrically grounded.

As shown in FIG. 1 a developing electrode is provided below the developer pool to face the drum about 3 mm therefrom, the length thereof in the peripheral direction being about 35 mm. Development without the developing electrode showed significant fogging (undesirable toner deposition) in the non-image areas, though the developed image showed sufficiently high image density and sufficiently uniform reproduction of blackout areas. The use of a grounded developing electrode provided similar results, while the application of a positive potential of 200 volts resulted in a remarkable decrease of fogging while retaining sufficiently high image density. Furthermore, by increasing the potential to 400 volts, the fogging was reduced to a scarcely recognizable level while the obtained image retained sufficiently high density and well reproduced blackout areas. With the application of 800 volts a certain decrease of image density in line image areas was observed together with a density decrease in the central portion of, blackout areas. These experiments were carried out at a peripheral speed of the drum of the about 15 cm/sec.

On the other hand, the conventional cascade process provided extremely low density in the central portion of blackout areas as well as considerably low density in line images, and it was necessary to reduce the peripheral speed as low as about 5 cm/sec in order to obtain the image density in line images comparable to that in the foregoing example. The developing process disclosed in this example was capable of providing sufficient image density even at a peripheral speed of 10 cm/sec, and capable of obtaining satisfactory image density and reproduction of blackout areas even at a speed of 20 cm/sec.

Continuous multiple copying was carried out with a voltage of +300 volts applied to the developing electrode 14 by means of an electric source shown in FIG. 2. In this case the entire copying device, including the developing device and the exposure mechanism, was 1 started by pressing the start button of the copying de-' vice. In this condition a positive potential is applied to the electrode 14 from the output terminal C as the electric power is supplied to the device and a switch SW2 is closed. When the exposure lamp is cut off at the completion of exposure for the final copy, the signal therefore simultaneously closes a switch Sw, to start a timer TIM A previously set at about 3 seconds, which is substantially equivalent to the time required for the latent image formed by the final exposure to pass through the developing device and to complete the developing process. After the 3 seconds have elapsed, a signal generated by the timer opens the switch SW and closes the switch SW to apply, through the output terminal C, to the developing electrode 14 a potential of -l00 volts. Further 15 seconds later, simultaneously with the ter mination of operation of entire copying device, the switches Sw, and SW3 are opened while the switch SW is closed to recover the original condition. This arrangement facilitates; l. automatic cleaning of the developing electrode after a series of continuous multiple copying and; 2. copying of a constantly low fogging level. The change of fogging level in this area is shown in FIG. 3. Although the fogging level gradually increases in continuous copying and is never reduced by the circuitry of FIG. 2 unless the copying is interrupted, this fact poses no practical problem since continuous copying of 300 to 500 copies seldom occurs in practice.

In another test, the developing electrode was grounded and then connected to a negative potential of 50 volts on each copying cycle in the course of continuous copying, and this method was more effective when compared with the above-mentioned example whenever the number of copies reached several hundred or more.

Although the process of this invention has thus far been explained in connection with a developing process utilizing a developer pool at the initial stage thereof, it is again to be noted that the process of this invention is by no means limited to this embodiment.

Numerous modifications of the invention will become apparent to one of ordinary skill in the art upon reading the foregoing disclosure. During such a reading it will be evident that this invention provides unique electrophotographic developing apparatus and method for accomplishing the objects and advantages herein stated.

What is claimed is:

1. Electrophotographic cascade developing apparatus for producing a predetermined number of copies of an original, said apparatus comprising a surface holding an electrostatic image of said original;

a developing electrode disposed in facing relationship a pre-determined spaced distance from said surface;

means for moving said surface with respect to said electrode;

means for flowing cascade developer including charged toner particles and carrier particles between said surface and said electrode to develop said latent image; means for applying to said electrode during said development a first potential for attracting said charged toner particles, said first potential being maintained on said electrode until said predetermined number of copies have been developed;

means for detecting the passage of that portion of said surface containing the developed image away from the space under the influence of said electrode;

means responsive to said detecting means for changing said first potential to a second potential which attracts said charged toner particles less than said first potential and facilitates the removal of developer particles which stick on the developing electrode;

means for maintaining said flow of said developer while said second potential is applied to said electrode.

2. Apparatus as in claim 1 where said potential changing means includes means for decreasing said first potential to said second potential.

3. Apparatus as in claim 1 where said potential charging means includes means for applying to said electrode said second potential which is of inverted polarity with respect to said first electric potential.

4. Apparatus as in claim 1 including means for establishing a pool of said developer in contact with said surface and above the opening into the space between said electrode and said surface.

5. Apparatus as in claim 4 where said pool establishing means includes means for establishing the length of said pool along the periphery of said surface so that said length is not less than the depth of said pool.

6. Apparatus as in claim 4 including means for replenishing the developer in said pool with the unused developer which passes through the space between said developing electrode and said surface. 

1. Electrophotographic cascade developing apparatus for producing a predetermined number of copies of an original, said apparatus comprising a surface holding an electrostatic image of said original; a developing electrode disposed in facing relationship a predetermined spaced distance from said surface; means for moving said surface with respect to said electrode; means for flowing cascade developer including charged toner particles and carrier particles between said surface and said electrode to develop said latent image; means for applying to said electrode during said development a first potential for attracting said charged toner particles, said first potential being maintained on said electrode until said predetermined number of copies have been developed; means for detecting the passage of that portion of said surface containing the developed image away from the space under the influence of said electrode; means responsive to said detecting means for changing said first potential to a second potential which attracts said charged toner particles less than said first potential and facilitates the removal of developer particles which stick on the developing electrode; means for maintaining said flow of said developer while said second potential is applied to said electrode.
 2. Apparatus as in claim 1 where said potential changing means includes means for decreasing said first potential to said second potential.
 3. Apparatus as in claim 1 where said potential charging means includes means for applying to said electrode said second potential which is of inverted polarity with respect to said first electric potential.
 4. Apparatus as in claim 1 including means for establishing a pool of said developer in contact with said surface and above the opening into the space between said electrode and said surface.
 5. Apparatus as in claim 4 where said pool establishing means includes means for establishing the length of said pool along the periphery of said surface so that said length is not less than the depth of said pool.
 6. Apparatus as in claim 4 including means for replenishing the developer in said pool with the unused developer which passes through the space between said developing electrode and said surface. 